Anodized magnesium and magnesium alloys

ABSTRACT

MAGNESIUM METAL OR MAGNESIUM ALLOYS ARE ANODIZED BY IMMERSING IN AN AQUEOUS BATH CONTAINING SODIUM HYDROXIDE AND SODIUM HEXAFLUOROTITANATE AND PASSING AN ALTERNATING CURRENT THROUGH THE BATH.

United States Patent Oflice 3,732,152 Patented May 8, 1 973 3,732,152 ANODIZED MAGNESIUM AND MAGNESIUM ALLOYS David L. Hawke, Metuchen, N.J., assignor to N L Industries, Inc., New York, N.Y. No Drawing. Filed July 13, 1971, Ser. No. 162,316 Int. Cl. C23b 9/06 U.S. Cl. 204-56 M 2 Claims ABSTRACT OF THE DISCLOSURE Magnesium metal or magnesium alloys are anodized by immersing in an aqueous bath containing sodium hydroxide and sodium hexafiuorotitanate and passing an alternating current through the bath.

BACKGROUND OF THE INVENTION Various methods have been used in the past to form protective films on magnesium or magnesium alloys through anodizing. Films formed by the methods in common use are inherently porous, and unless sealed by supplementary coatings, they fail to retard, and may in fact stimulate, pitting corrosion in saline environments.

SUMMARY OF THE INSTANT INVENTION A method for anodizing the surface of magnesium metal or magnesium alloys which comprises immersing said metal or alloys in an aqueous bath co 'ntaining sodium hydroxide and sodium hexafluorotitanate and passing an alternating current through said bath-to form the anodized coating on said metal or alloy, the temperature of said bath being held from room temperature to 80 F. during the anodizing treatment.

The amount of sodium hydroxide employed in the bath is from 10-50 g.p.l. and the amount of sodium hexafluorotitanate is from 5-15 g.p.l.

In addition it has also been found that sodium vanadate (NavO -nH O) in amount of 5-15 g.p.l. tends to enhance the anodizing effect.

At least two metallic specimens are employed in the bath. Both of these specimens act as electrodes. More than two metallic specimens may be used provided they are in electrical contact with either of these metallic electrodes.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In carrying out the instant invention it is desirable to use dilute solutions in the bath. It has been found that good anodizing coatings are obtained when the aqueous bath contains only -50 g.p.l. NaOH and from 5-15 g.p.l. sodium hexafluorotitanate. When such concentrations are used in the bath, an anodizing coating appears to form within one minute after the alternating current is passed through the bath. The voltage rises rapidly to about 120 while the current falls to a low value. The anodized film is light gray in color and appears to be about 1 micron in thickness.

The bath is held at a temperature below 80 F. by use of external cooling. The alternating current is regulated by means of an autotransformer. In carrying out the anodizing procedure, the occurrence and completeness of the film formation is reflected by an almost immediate rise in the voltage to maintain the current flow, which results in a nearly complete shut-off of the current within a few minutes with a terminal voltage of 110-120 volts.

The protective value of the anodized coating was determined by using a standard Salt Fog TestASTM- B117-64.

Average corrosion rates were calculated from the weight loss. Maximum pit depths were estimated by means of a needle point depth gauge.

Magnesium alloys may be anodized in the same manner as that used to anodize magnesium metal and therefore are included in the term magnesium metal.

In order to describe more fully the instant invention, the following examples are presented:

EXAMPLE 1 Two magnesium metal panels were suspended as electrodes in an aqueous bath which contained 25 g.p.l. NaOH and 6.25 g.p.l. Na TiF The bath was provided with external means to cool the liquid. An initial alternating current of 20 amps per sq. ft. and 5 volts was passed through the bath. The magnesium metal panels were coated with an anodized continuous coating which had a light gray color. The current dropped to nearly zero within 1 or 2 minutes with a rapid rise in voltage to 110-120 volts. This indicated a completeness of the anodizing coating. To insure completeness these conditions were held for 12 minutes. During the anodizing the bath temperature was held at -80 F.

The anodized panels were then tested in a standard Salt Fog Test and the test results obtained were measured as the rate of corrosion and the depth rate of pitting. The results are recorded as follows:

Salt fog test py Corrosion rate 0.009 Depth of pitting 0.4-0.8

EXAMPLE 2 In this run the procedure described in Example 1 was repeated except that sodium vanadate was also used in the bath in addition to sodium hexafiuorotitanate and sodium hydroxide.

The bath contained the following composition:

G.p.l. NaOH 25 NfigTIFs NaVO -nH O 6.25

The anodized magnesium panels were tested in the same standard Salt Fog Test and the results are recorded as follows:

Corrosion rate 0.00 28 Depth of pitting 0.2

These corrosion figures are well below the corrosion results obtained by using the prior art anodizing techniques which employ the well known standard alkaline anodizing baths.

While this invention has been described and illustrated by the examples shown, it is not intended to be strictly limited thereto, and other variations and modifications may be employed within the scope of the following claims.

1. A method for anodizing the surface of magnesium metal which consists essentially of passing an alternating current through an aqueous bath containing at least two magnesium metal specimens; said metal being the electrodes in said bath, said bath comprising sodium hydroxide and sodium hexafiuorotitanate, the amount of sodium hydroxide in said bath being from 10 to 50 g.p.l. and the amount of sodium hexafiuorotitanate being from 5 to 15 g.p.l.

(2. Method according to claim 1 in which said hath also contains sodium vanadate in an amount from 5 to 15 g.p.l.

References Cited UNITED STATES PATENTS Loose et a1. 20456 M X Evangelides 204-56 M X Richaud 20432 Wright et al 204-56 R Fruchtnicht 204-46 M 10 FREDERICK C. EDMUNDSON, Primary Examiner 

